It is well known that the performance of castable compositions can be improved by the incorporation of reinforcing elements. For example, reinforcing elements are incorporated into cementitous compositions such as mortar and concrete to improve their strength or to reduce surface cracking tendencies. Reinforcing elements encompass a broad range of shapes, but they typically share an elongate structure (ie., the dimension of a major axis is significantly larger than the dimension of the minor axis). The elongate structures include rod-like filaments having substantially round cross-sections, flat strips having a helically twisted structure or made with a varying cross-section to reduce pull-out tendencies, or cable-like constructions based on twisted constructions of rod-like filaments. Materials used as reinforcing elements include metals, synthetic polymeric materials, and naturally occurring materials.
In cementitous compositions (ie. mortar or concrete), it is known that the impact strength, flexural strength and toughness of the material improves with increasing percentages of the reinforcing elements in the composition. There is, however, a practical limit to the level of reinforcing elements that can be introduced as above this limit, the reinforcing elements tend to entangle with each other, forming balls or other undesirable conglomerates.
Proper selection of criteria relating to the length and diameter (eg. aspect ratio) of the reinforcing elements can help reduce their tendency to agglomerate. Achieving improved mixability by this technique is difficult, however, as these types of changes negatively impact the reinforcing capabilities of the elements.
The method of delivering reinforcing elements into cementitous compositions also plays a major role in realizing high incorporation levels of reinforcing elements. For example, high aspect ratio reinforcing elements must be introduced into cementitous compositions as substantially individual fibers and in very low concentrations over an extended period of time with very good mixing if high volume concentrations of reinforcing elements are desired.
Rod shaped reinforcing elements can become entangled and/or agglomerated in their shipping containers as a result of the vibration and jostling encountered during shipping. Agglomerated elements are not readily pourable from their shipping containers and cannot be added to cementitous compositions until the agglomerates have been broken up into individual elements. Various mechanical and pneumatic techniques have been employed to break-up the agglomerated masses and introduce the individual elements into the cementitous mix as a rain of elements. U.S. Pat. No. 4,121,943 (Akazawa et al.) describes a machine designed to separate reinforcing fibers into separate units prior to introducing them into the cementitous mix.
An alternative approach to incorporating high levels of reinforcing elements into cementitous compositions is described in U.S. Pat. No. 3,716,386 (Kempster), where fibers are treated with a high viscosity, friction reducing substance prior to the reinforcing elements being introduced into the mix.
A factor common to a number of recent approaches to achieving high incorporation levels of high aspect ratio elongate reinforcing elements (ie. fibers) into cementitous compositions is to introduce the fibers into the compositions in an organized, aligned array that, on mixing, slowly releases the fibers in an aligned array. Fibers released into cementitous compositions in this manner experience fewer fiber-fiber interactions and subsequently show less tendency toward agglomeration or balling as compared to fibers introduced into the compositions in a totally random orientation. U.S. Pat. Nos. 4,224,377 and 4,314,853 (Moens) describe a reinforcing member comprising a plurality of wire elements which are united by a binder which loses its binding ability during the mixing process, and a process for incorporating these members into cementitous compositions. U.S. Pat. No. 4,414,030 (Restrepo) describes reinforcing elements for cementitous compositions comprising ribbons of fibrillated polyolefin film. The fibrillated fibers are maintained in an aligned configuration by inter-fiber fibrils which are broken by mechanical mixing, thereby releasing individual fibers into the cementitous composition in an aligned configuration.
Another alternative to introducing fibers into cementitous compositions in an aligned configuration is described in Fiber Reinforced Cements and Concretes, edited by R. N. Swamy and B. Barr, Elsvier Applied Science, New York, 1989. pp 316-325. This processes utilizes short fibrillated polypropylene fiber strips which are twisted to produce a cable-like structure to deliver the fibers to cementitous compositions. The mechanical mixing action of processing the cementitous composition breaks the cable-like structure apart, releasing individual fibrillated film fibers into the mix in an aligned manner.